Implantable medical devices may be employed in various applications. For example, an implantable cardiac device may perform one or more functions including sensing signals generated in the heart, pacing the heart to maintain regular contractions, and providing defibrillation shocks to the heart. Similarly, an implantable stimulation device may be used to apply stimulation signals to a patient's muscular tissue, neurological system, or some other area of the patient's body.
In practice, there may be a need to communicate with an implantable medical device after it has been implanted in a patient. For example, an external monitoring device located in a person's home, a doctor's office, a clinic, or some other suitable location may be used to retrieve information collected by and stored on the implanted medical device. In the case of an implantable cardiac device such information may include sensed cardiac activity that a treating physician may wish to analyze to determine the relative health of the patient. Similarly, an external programming device located in any of the above locations may be used by a treating physician to change the operating parameters of the implanted medical device. Such parameters may include, for example, the timing or magnitude of stimulation pulses generated by the implanted medical device.
In a typical implementation, an implantable medical device utilizes radio frequency (“RF”) telemetry to communicate with an external device. Thus, the implantable medical device may include an RF transceiver that is adapted to transmit and receive the necessary RF signals. In such an implementation, however, it is generally desirable to leave the transceiver in a powered-off or low power state as much as possible since a transceiver typically consumes a relatively large amount of power. Here, it should be appreciated that the replacement of the battery in an implanted medical device involves a surgical procedure. Hence, long battery life is an important aspect of an implantable medical device.
Some types of implantable medical devices employ a wake-up scheme whereby an implantable medical device will periodically turn on its transceiver (e.g., its receiver) to determine whether an external device is attempting to communicate with the implantable medical device. For example, whenever an external device wishes to establish communication with an implantable device the external device periodically transmits polling messages (e.g., connection requests) over one or more designated RF channels. Each polling message may include information relating to establishing the communication such as, for example, an identifier that uniquely identifies the implantable medical device.
Every time the transceiver is turned on it may then conduct a scan to determine whether the external device is transmitting polling messages. This may involve, for example, performing an ID scan that checks each RF channel for any messages that include the identifier of that particular implantable medical device. In the event such a signal is detected, the implantable medical device transmits an appropriate signal to establish communication with the external device.
In practice, a transceiver of an implantable medical device may still consume a relatively significant amount of power even when a conventional wake-up scheme is used. For example, scanning each RF channel for polling messages consumes a relatively large amount of power since the RF receiver must be enabled for a relatively long period of time to perform the search. Moreover, it is also desirable that an implantable medical device respond to polling messages within a relatively short period of time. For example, a treating physician would normally not wish to wait several minutes to establish communication with the implanted medical device. To achieve such a quick response time, however, the implantable medical device may need to perform its scans at relatively frequent intervals. Hence, a relatively large amount of power may be consumed due to the frequency of such scans.
Some forms of wake-up schemes employ staged detection to further reduce the amount of power consumed by the implantable medical device. Here, the implantable medical device may employ an early detection stage that triggers a more refined detection stage that analyzes any detected signals to determine whether the signals are from an external device that is attempting to establish communication with the implantable medical device. For example, one technique employs an initial energy “sniff” in the radio frequency channel or channels of interest. By starting a detection search with this low-level and relatively coarse energy assessment stage, the implantable medical device may bypass the relatively high power stage scan whenever a radio frequency signal is not present. In practice, the absence of such an RF signal will be the case for most of the lifetime of the implantable medical device. Hence, additional power savings may be achieved with this type of wake-up scheme.